Films of superconducting material, with thicknesses of from about 0.1 to about 500 microns, have been proposed for use in electronic circuits and superconducting devices. See, for example, an article by Peter E. Norris et al. entitled "In-situ thin films by MOCVD," Superconducting Industry, Vol. 3, No. 1, Spring, 1990.
The high Tc superconductors currently available are ceramic materials. It is known that ceramic materials may be fabricated into film by evaporation or by sputtering methods; however, both of these methods are usually conducted under vacuum. See, e.g., B. Oh et al., "Critical current densities and transport in superconducting YBaCuO films made by electron beam coevaporation," Applied Physics Letters 51, 852 (1987); M. Hong et al., "Superconducting Y-Ba-Cu-O oxide films by sputtering," Applied Physics Letters 51, 694 (1987); D. Dijkkamp et al., "Preparation of Y-Ba-Cu oxide superconductor thin films using pulsed laser evaporation from high Tc bulk material," Applied Physics Letters 51, 619 (1987); and S. Witanachchi et al., "Deposition of superconducting Y-Ba-Cu-O films at 400 degrees centigrade without postannealing," Applied Physics Letters 53, 234 (1988).
The aforementioned prior art techniques for making films consisting of superconductive material all require the use of a reduced pressure environment during the vapor deposition step; in general, a vacuum of from 10.sup.-3 to 10.sup.-9 Torr is required. Such processes are not suitable for large scale production of the superconductive films. A substantial amount of energy, time, and money is required for a vacuum system and its operation; and such a system is not always suitable for large scale production. In addition, the size of the superconductive film which can be made by the vacuum deposition processes is limited by the size of the vacuum chamber used.
It is known that superconductive films with thicknesses in excess of 100 microns may be prepared by a tape-casting process in which vacuum is not required. See, e.g., M. Ishii et al., "Fabrication of superconducting YBaCuO films by a tape casting method," Japanese Applied Physics 26, L1959 (1987). However, this tape casting process does not routinely produce superconductive films with good adhesion properties and adequate film orientations.
It is also known that superconductive films may be prepared by a process involving plasma spraying of powders onto a surface through a high-temperature gas plasma; in this process, vacuum might not be required. See, for example, the article by M. Sayer et al. entitled "Ceramic Thin Films: Fabrication and Applications," Science, Volume 247, Mar. 2, 1990, pages 1056 to 1060. However, this powder process generally produces films which contain a substantial amount of inhomogeneity; see, e.g., J. J. Cuomo et al., "Large area plasma spray deposited superconducting YBaCuO thick films," Advanced Ceramic Materials 2, 442 (1987) and W. T. Elam et al., "Plasma Sprayed High Tc Superconductors," Advanced Ceramic Materials, Volume 2, 411 (1987). Furthermore, due to the inhomogeneity produced by the process, the films may not be superconductive. See a paper by T. K. Vethanayagam et al., "Atmospheric Plasma Vapor Deposition of Ba-Y-Cu-Oxide and Bi-Sr-Ca-Cu-Oxide Thin Films," Proceedings of the National Thermal Spray Conference, October, 1988, Cincinnati, Ohio.
Attempts have been made to overcome the problems associated with the powder plasma technique by preparing films by a mist microwave plasma method under vacuum. Such a process is disclosed by A. Koukitu et al., Japanese Journal of Applied Physics, Volume 28, Number 7, July, 1989, pages L1212-L1213. In the Koukitu process, an aqueous solution of yttrium nitrate, barium nitrate, and copper nitrate was misted by an ultrasonic generator, the mist was injected into a quartz reactor where it was subjected to microwave radiation at a frequency of 2.45 Gigaherz, and plasma thus formed from such mist was deposited onto a substrate in the quartz reactor. The deposition step was conducted under a vacuum of 50 Torr.
The Koukitu process is not suitable for large scale production of superconductive films. In the first place, it requires the use of reduced pressure during the deposition step, with all of the disadvantages associated therewith discussed above. In the second place, the deposition must take place in a quartz reactor, thereby limiting the size of the film which may be obtained. In the third place, the Koukitu process requires microwave energy, which often is more expensive and difficult to furnish than other forms of energy, such as radio frequency energy. In the fourth place, the Koukitu process proceeds very slowly; it is disclosed that the growth rate of the films produced by the process is about 0.15 microns per hour.
Koukitu et al. did not specifically describe the dimensions of the deposition chamber they used. However, the Koukitu et al. paper did disclose that their deposition system was " . . . similar to that of the microwave plasma CVD system which is commonly used for the preparation of diamond films . . . ," and it cited a paper by M. Kamo et al., "Diamond Synthesis from Gas Phase in Microwave Plasma," Journal of Crystal Growth 62 (1983), pages 642-644 (see page L1212 of Koukitu et al.). The Kamo et al. reference discloses a deposition apparatus comprised of a ". . . silica glass tube 40 mm in diameter . . . ."
Thus, with the deposition apparatus of both Koukitu et al. and Kamo et al., the maximum area of a substrate which may be contained by the deposition chamber is about 12.6 square centimeters.
The deposition process of Koukitu et al. is substantially slower than the laser deposition process described in the aforementioned S. Witanachchi et al. paper; the latter process forms superconducting films on a substrate with a surface area of 2 square centimeters at a rate of at least about 1 microns per hour.
The Koukitu paper does not disclose either the morphological surface properties of the films produced by its process or their color.
It is an object of this invention to provide a process for producing superconductive films or coatings that will produce such materials at a rate of at least 1 micron per minute on a substrate with a surface area of at least about 30 square centimeters.
It is another object of this invention to provide a process for producing superconductive films or coatings that is able to readily produce such materials in large sizes and/or complicated shapes.
It is yet another object of this invention to provide a process for producing superconductive films or coatings that are substantially homogeneous.
It is yet another object of this invention to provide a process for the production of superconductive films or coatings that may be conducted under atmospheric pressure.
It is yet another object of this invention to provide a process for the production of superconductive films or coatings which is relatively economical and flexible.
It is yet another object of this invention to provide a process for producing superconductive films or coatings which contain a large number of relatively small grains.
It is another object of this invention to provide a process for coating a moving object with superconductive material which coats the object as it is being formed.